1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device and, more particularly, to a method of manufacturing a semiconductor device having a capacitor portion.
2. Description of the Prior Art
For semiconductor devices such as DRAMs, integration of a semiconductor device having a capacitor as a constituent element is progressing year by year.
For high integration, the wiring and circuit elements must be micropatterned. However, when the stored charge amount corresponding to a signal is reduced by micropatterning, an erroneous operation (software error) of the memory is caused by, e.g., radioactive rays such as .alpha.-rays. To solve this problem, a technique of reducing the thickness of the dielectric film of the capacitor to increase the capacitance value of the memory is used.
In a semiconductor device manufacturing method disclosed in Japanese Unexamined Patent Publication No. 2-16763 (to be referred to as prior art 1 reference hereinafter), the polysilicon film surface of a lower electrode is nitrided to convert a spontaneous oxide film on the surface into a silicon nitride film. The silicon nitride film is grown by low-pressure chemical vapor deposition (to be referred to as LP-CVD hereinafter). With this technique, the growth of the spontaneous oxide film on the polysilicon film surface can be suppressed when the wafer is fed into the LP-CVD furnace, and a high capacitance value can be obtained.
FIGS. 1A to 1E are sectional views showing steps in manufacturing the semiconductor device of the prior art 1 reference. As shown in FIG. 1A, a silicon oxide film 2 is formed on a silicon substrate 1, and a polysilicon film 9 as a lower electrode is deposited on the resultant structure. The polysilicon film is doped with an impurity such as phosphorus by ion implantation or diffusion. When the resultant structure is left to stand at room temperature, a spontaneous oxide film 10 is formed on the surface of the polysilicon film 9, as shown in FIG. 1B.
As shown in FIG. 1C, the spontaneous oxide film 10 formed on the surface of the polysilicon film 9 is converted into a silicon nitride film 11 by rapid thermal nitridation (to be referred to as RTN hereinafter).
As shown in FIG. 1D, a silicon nitride film 12 is deposited on the silicon nitride film 11 by LP-CVD. The surface of the silicon nitride film 12 is oxidized to form a silicon oxide film 13. The dielectric film of a capacitor is constituted by the silicon nitride film 11, the silicon nitride film 12, and the silicon oxide film 13. As shown in FIG. 1E, a polysilicon film 14 as an upper electrode is formed on the silicon oxide film 13.
In a semiconductor device manufacturing method disclosed in Japanese Unexamined Patent Publication No. 5-190769 (to be referred to as prior art 2 reference hereinafter), an amorphous silicon film is formed on a polysilicon film as a lower electrode. After nitrogen atoms are ion-implanted, the amorphous silicon film is nitrided by RTN to form a silicon nitride film. With this technique, a high-quality thin dielectric film is realized.
FIGS. 2A to 2E are sectional views showing steps in manufacturing the semiconductor device of the prior art 2 reference. As shown in FIG. 2A, a silicon oxide film 2 is formed on a silicon substrate 1, and a polysilicon film is deposited on the resultant structure. The polysilicon film is doped with an impurity such as phosphorus by ion implantation or diffusion and patterned into the shape of a lower electrode 3. As shown in FIG. 2B, an amorphous silicon film 4 is formed on the surface of the polysilicon film by LP-CVD.
As shown in FIG. 2C, nitrogen atoms are implanted into the amorphous silicon film. As shown in FIG. 2D, the amorphous silicon film is nitrided by RTN and then oxidized to form a nitrooxide film 15 on the silicon nitride film. As shown in FIG. 2E, a polysilicon film is formed on the nitrooxide film 15 as a dielectric film. An impurity such as phosphorus is diffused, and the polysilicon film is patterned into the shape of an upper electrode 8.
The above-described prior arts have the following problems.
In the prior art 1 reference, the silicon nitride film is formed on the polysilicon film as a lower electrode by RTN. However, no perfect silicon nitride film is formed on a silicon-oxide-based insulating film, i.e., the interlayer adjacent to the lower electrode.
In FIGS. 1A to 1E, when the silicon nitride film 12 is to be formed by LP-CVD, the growth rate at the first stage of formation changes on the silicon nitride film 11 formed on the surface of the lower electrode 9 by RTN and on the silicon-oxide-based insulating film adjacent to the lower electrode. Since the thickness of the silicon nitride film 12 on the surface of the lower electrode is made different from that on the insulating interlayer, a leakage current easily flows.
FIG. 3 shows the relationship between the growth time and growth thickness of the silicon nitride film on the silicon nitride film and on the silicon-oxide-based insulating film. The growth is delayed on the interlayer, so a thickness difference of about 2.5 nm is generated for the same growth time.
As a result, the thickness of the silicon nitride film on the lower electrode 3 is made different from that on the silicon oxide film 2 as an interlayer, as shown in FIG. 4, resulting in a breakdown failure or leakage current.
More specifically, in FIG. 4, the lower electrode 3 is formed of the polysilicon film 9 shown in FIGS. 1A to 1E, and the upper electrode 8 is formed of the polysilicon film 14 shown in FIG. 1E. The surface of the silicon oxide film 2 serving as an interlayer or field insulating film is adjacent to the patterned lower electrode 3. A dielectric film 16 in FIG. 4, i.e., the capacitive insulating film 16 of a capacitor is constituted by the silicon nitride film 11, the silicon nitride film 12, and the silicon oxide film 13 shown in FIGS. 1D and 1E. The most portion of this dielectric film 16 is constituted by the silicon nitride film 12 formed by CVD. That is, the thickness of the dielectric film 16 is dominantly determined on the basis of the thickness of the silicon nitride film 12.
The spontaneous oxide film 10 formed on the surface of the lower electrode, i.e., the polysilicon film 9 can be converted into the silicon nitride film 11 by the process of RTN shown in FIG. 1C because the spontaneous oxide film 10 is very thin. However, no silicon nitride film is formed on the silicon oxide film 2 having the necessary thickness of an interlayer or field insulating film. More specifically, although the surface of the silicon oxide film 2 is nitrided, the resultant film contains much oxygen and therefore has a strong attribute as silicon oxide.
When the silicon nitride film 12 is to be formed by LP-CVD in the process shown in FIG. 1C, the growth thickness changes, as shown in FIG. 3. As shown in FIG. 4, the dielectric film 16 formed on the surface (upper and side surfaces) of the lower electrode 3 becomes thick, and the dielectric film 16 formed on the silicon oxide film 2 becomes thin. For this reason, a constricted portion is formed in the silicon nitride film 12 at the lower end portion of the lower electrode 3, i.e., a constricted portion 17 is formed in the dielectric film 16.
In this state, a leakage current easily flows between the upper electrode 8 and the lower electrode 3 through this constricted portion 17, and a breakdown failure easily occurs at this portion.
This problem arises for a capacitor for which the dielectric film 16 must be made thin, i.e., the silicon nitride film 12 must be made thin. If the design allows a thick silicon nitride film 12, a sufficient thickness free from the problem of voltage in use can be ensured at any portion even when the thickness changes due to the delay in chemical vapor deposition.
To prevent the breakdown failure or leakage current, the silicon nitride film must be thick. The thickness of the silicon nitride film cannot be 7 nm or less. The thickness of the dielectric film cannot be 5 nm or less as a thickness converted into the silicon oxide film. Therefore, a capacitor having a large capacitance value can hardly be obtained.
In the prior art 2 reference, the dielectric film is constituted only by the silicon nitride film which is formed by implanting nitrogen atoms into the amorphous silicon film and performing RTN. The dielectric film can hardly serve as a perfect insulating film. For this reason, the upper electrode and the lower electrode are easily short-circuited, and a high-quality dielectric film can hardly be formed.
Even when the silicon nitride film is made thick by increasing the thickness of the amorphous silicon film to improve the insulating properties, only a dielectric film having imperfect insulating properties is formed because the thick amorphous silicon film is not easily nitrided. Therefore, a leakage current easily flows between the upper and lower electrodes.
As described above, in the prior arts, the dielectric film serving as a capacitive insulating film cannot be made thin and therefore cannot cope with micropatterning of the device.